Patent Documents 1 and 2, for example, disclose a fuel supply devices that widen the dynamic range of fuel injection amount by changing fuel pressure.
The fuel supply device of Patent Document 1 supplies pressurized fuel from an end of a fuel branch pipe, which serves as a fuel distribution pipe, to an internal combustion engine through a supply line and returns excessive fuel from the other end of the fuel branch pipe to a fuel tank through a return line. The return line is branched into a line that has a low pressure regulating valve and an electromagnetic valve and a line that has a high pressure regulating valve only.
The fuel supply device of Patent Document 2 also supplies pressurized fuel from an end of a fuel gallery, which serves as a fuel distribution pipe, to the engine through a supply line. However, the fuel supply device does not include a return line through which excessive fuel is returned to a fuel tank. The supply line is branched into a line having a low pressure regulating valve and an electromagnetic valve and a line including a high pressure regulating valve only.
In the techniques of Patent Documents 1 and 2, the pressure of fuel in the fuel distribution pipe (the fuel branch pipe or the fuel gallery) rises when the electromagnetic valve closes and lowers when the electromagnetic valve opens. The level of the fuel pressure is regulated by controlling operation of the electromagnetic valve in accordance with the operating state of the engine (such as the throttle opening degree or the load on the engine).
In the device of Patent Document 1, the return line is connected to the end opposite to the end of the fuel branch pipe to which the supply line is connected. Accordingly, when the fuel injection amount is small as in a low load state or a low rotation state of the engine or fuel cutoff is carried out, a large amount of fuel that has been heated through the fuel branch pipe returns to the fuel tank via the return line, thus raising the temperature in the fuel tank.
Since the return line is not provided in the device of Patent Document 2, the heated fuel does not return to the fuel tank so that the fuel tank is not heated. However, since the fuel gallery cannot be cooled by the fuel that passes there through, the fuel may be injected through the fuel injection valve with fuel vapor generated in the fuel gallery if the engine is started at high temperature. If the fuel vapor is injected from the fuel injection valve, the amount of fuel falls short and hampers starting of the engine, lowers control the accuracy of the air-fuel ratio, or degrades the performance of the engine due to insufficient output. Also, if catalyst bed temperature control is performed on a catalyst provided in the exhaust system of the engine, the exhaust gas cannot be sufficiently enriched. Thus, the catalyst may be heated and melted.    Patent Document 1: Japanese Laid-Open Patent Document No. 5-59976    Patent Document 2: Japanese Laid-Open Patent Document No. 2001-221085